@Vitaliy_Kiselev
I dismantled the gear box, but wasn't able to pull the focus wheel off. The fault is not the gear box but the shaft that attaches to the focus wheel. Any suggestion on how to remove the focus wheel?

Just for interest, the geared shaft that the toothed wheel fits on is aluminum. The internal piece that connects to the focus wheel is a plastic gear on an aluminum shaft.

To clarify, this isn't the D&B and I purchased from Ebay seller "hermeshine". Quality control could be different. One thing I can say is that they were very easy to deal with, prompt reply's and a resolution was agreed on in light of the FF problem.